Theme development

I'm new to Drupal, however I've designed other sites in the past using various applications. I've been poking around and learning what Drupal 8 has to offer and trying to use just Drupal 8 and a text editor. From what I have discovered there are several things that are not accessible to edit from the Drupal 8 "toolkit".

i have created a module.
in my module i have created a form. Form submits data successfully in database.
i am getting data from database in a controller file. till that point everything is working fine.
Now i want to display data in my twig file.
How it is possible to display data from database in a twig file.?

What theme do you recommend for creating sub-theme (I prefer simple css or less)? Im not very experienced, I know css, less, html, basic js. I use grunt. I'm using bootstrap at the moment but don't use its functionalities so there is no point using BS and it has a lot of code that makes my work complicated. I want to make simple website, gallery, info, contact, news etc. If possible i'm looking for a responsive theme that won't be deprecated in future Drupal versions.

I'm not getting how to create a sub-theme. I am just hacking Bartik. But I was trying to do it in a child theme. All I want is to add one style.css file so I can make some overrides to the default Bartik theme. Here is my info and libraries file.  I have my style.css saved in the same theme folder as the info and libraries files. 

Thanks for any help. It seems right, but I can't get it to load.

Hello.

New to twig adn drupal.

I am trying to check if a field has value and I use this code

{% if(content.field_info_simeio_synantisis) %}

which is alway true.

Shouldn't this be false from empty fields?

Hello,

I am new to twig and Drupal.

I have an Image Field that can take multiple items.

The following code in twig laways retuns 2 even if there is not image submitted. 

{% for i in 0..node.field_periodic_image_gallery|length-1 %}

Souldn  't this return 0 when I have no image submitted?